Unified power architecture

ABSTRACT

A unified power architecture for generating high frequency switched power to a load is provided which includes a number of input branches, each input branch receiving direct current (DC) power from a power source. Each input branch includes a switch device coupled to a storage device for generating input pulses, and a transformer including a primary winding set receiving the input pulses. An output section is provided which includes a plurality of windings around a secondary of each transformer of each of the input branches, the output section generating an output pulse including components of each of the input pulses.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to co-pending U.S. patent applicationSer. No. ______, filed Jun. 6, 2001 (on even date herewith), AttorneyDocket No. 2001P10382US for “UNIFIED POWER ARCHITECTURE WITH DYNAMICRESET” the content of which is incorporated by reference herein for allpurposes.

FIELD OF THE INVENTION

[0002] The present invention relates to power supplies, and moreparticularly, to an improved power supply for use in powering linearaccelerators, and similar devices.

BACKGROUND OF THE INVENTION

[0003] Linear accelerators are used in a wide variety of applications.One important application is use in radiation therapy devices for thetreatment of patients. In such an application, the linear accelerator isused to generate a high energy radiation beam for therapy. The highenergy radiation beam is directed at a treatment zone (such as acancerous tumor) on a patient to provide a selected dose of therapeuticradiation pursuant to a treatment plan prescribed by, e.g., anoncologist.

[0004] Typically, electron guns are used to generate electron beamssupplied to the linear accelerator. A high energy beam is then createdusing a high frequency source (such as a magnetron or klystron) tosupply radio frequency signals for the generation of an electromagneticfield. This electromagnetic field accelerates electrons in theaccelerator, creating a high energy beam. The high energy beam can be anelectron or photon (X-ray) beam.

[0005] An important component of these radiation therapy devices is thepower system which drives the electron gun and the high frequencysources. Typically, a radiation therapy device may have one or morepower systems, one to provide power to drive the electronic gun and oneto provide high frequency power to drive either a magnetron, klystron,or other high frequency source. There is typically a different designfor each power system, and often a different design is used fordifferent high frequency sources. These power systems are used in anextremely unforgiving environment requiring high accuracy, reliability,maintainability and safety in a relatively small footprint all at a lowcost of operation.

[0006] Highly accurate power supplies, delivering accurate pulsed powerat a prescribed frequency are needed. Treatment therapies, typicallyprescribed for each patient by an oncologist, require accurate deliveryof prescribed doses of therapeutic radiation. Accurate control of thepower system driving the magnetron, klystron, and/or electron gun isessential to this accurate delivery of radiation.

[0007] The overall reliability of radiation therapy devices is animportant concern to users of the devices and to patients. Typically,radiation therapy devices are very expensive units operated by hospitalsand treatment centers (generically referred to herein simply as“hospitals”) to treat life-threatening ailments such as cancer.Hospitals often can only afford one or two radiation therapy devices andtherefore demand very high reliability in their operation. Because oftheir high cost, hospitals often run these devices at a brisk pace,scheduling treatments throughout every working hour of the week. Failureof the device is potentially devastating to both the hospital (in termsof revenue, scheduling, and patient care) as well as to patients whohave a real and pressing need for uninterrupted treatment.

[0008] There is also a need for radiation therapy devices which areeasily maintained. Electronic components do not last forever.Eventually, components require maintenance and/or replacement. Whenmaintenance or replacement is required it is desirable to providecomponents which are easily and quickly maintained and installed byrelatively unskilled workers.

[0009] The environment for these radiation therapy devices is made evenmore difficult due to space and power consumption constraints imposed byhospitals. Many hospitals can only install radiation therapy deviceswhich occupy a relatively small amount of space. Other hospitals requireseveral radiation therapy devices to satisfy the treatment needs oftheir patients, but can only install several devices if each of theirfootprints is small.

[0010] Existing power systems for linear accelerators in radiationtherapy devices do not necessarily meet these needs for high accuracy,reliability, maintainability, and safety in a small footprint and at alow cost of operation. Many existing power systems for linearaccelerators are large, heavy devices that significantly increase thecost and size of the radiation therapy system. One typical power systemutilizes a high voltage transformer/rectifier system to generate a 21 kVDC power source from a conventional three-phase 208 V power source. Thehigh voltage DC source is then used to generate a 15 kV pulse that isconverted to the required 150 kV pulse via a high voltage pulsetransformer. The high voltage transformer/rectifier assembly typicallyweighs 500 lbs. and occupies eight cubic feet. As a result, the powersupply must be housed in a separate cabinet from the linear accelerator.In addition to increasing the floor space needed to house theaccelerator system, this additional cabinet requires special powertransmission lines to couple the klystron output to the linearaccelerator which further increases the cost and complexity of thesystem. Finally, the sheer weight of the system increases the cost ofshipping.

[0011] Many existing power systems utilize a pulse forming network and aswitch tube known as hydrogen thyratron. A thyratron is a low pressuregas device with a thermionic cathode. Over time, the cathode depletesitself. Thus, a thyratron has an inherent wear out mechanism. Morerecently, solid state power systems have been proposed. However, many ofthese systems utilize semiconductor controlled rectifiers (SCRs) tomodulate the high voltage pulses needed to drive klystrons ormagnetrons. Current SCRs tend to wear out relatively quickly under theseconditions.

[0012] It would be advantageous to provide a method and apparatus thatovercame the drawbacks of the prior art. In particular, it would bedesirable to provide a solid state power architecture which greaterreliability and maintainability which provides highly accurate pulsedpower to a variety of different loads. Preferably, the powerarchitecture achieves fast output pulse rise times in a modulararchitecture in a cost effective package taking up relatively littlespace.

SUMMARY OF THE INVENTION

[0013] To alleviate the problems inherent in the prior art, embodimentsof the present invention provide a unified power architecture suitablefor powering devices requiring high voltage pulsed power, such asklystrons, magnetrons, or the like.

[0014] In one embodiment of the present invention, a unified powerarchitecture for generating high frequency switched power to a load isprovided which includes a number of input branches, each input branchreceiving direct current (DC) power from a power source. Each inputbranch includes a switch device coupled to a storage device forgenerating input pulses, and a transformer including a primary windingset receiving the input pulses. An output section is provided whichincludes a secondary winding set around a secondary of each transformerof each of the input branches, the output section generating an outputpulse including components of each of the input pulses.

[0015] According to one embodiment of the present invention, the outputpulse is provided to a droop compensation circuit which ensures that theoutput pulse is of a good quality shape.

[0016] According to another embodiment of the present invention, thenumber of input branches is selected based on the requirements of theload to be driven. In one embodiment, two input branches are provided todrive a magnetron, while five input branches are provided to drive aklystron. In one embodiment, portions of each input branch is formed ona separate printed circuit board (PCB). In one embodiment, each inputbranch is interchangeable. The result is a unified power supply which ishighly reliable, easily maintained, modular, accurate, all with a lowcost of operation in a small footprint.

[0017] In one embodiment, the switch of each input branch is performedusing an Insulated Gate Bipolar Transistor (IGBT) operatively controlledby a control device.

[0018] With these and other advantages and features of the inventionthat will become hereinafter apparent, the nature of the invention maybe more clearly understood by reference to the following detaileddescription of the invention, the appended claims and to the severaldrawings attached herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a diagram of a radiation treatment device and treatmentconsole in which the power architecture of the present invention may beused;

[0020]FIG. 2 is a block diagram of a linear accelerator which may beused in the radiation treatment device of FIG. 1 according to anembodiment of the present invention;

[0021]FIG. 3 is a block diagram of the power architecture according toan embodiment of the present invention;

[0022]FIG. 4 is a further block diagram of the power architectureaccording to an embodiment of the present invention;

[0023]FIG. 5 is a schematic diagram of a portion of the powerarchitecture according to an embodiment of the present invention;

[0024]FIG. 6 is a schematic diagram of a droop compensation circuitaccording to an embodiment of the present invention;

[0025]FIG. 7A is a diagram depicting an output pulse without droopcompensation; and

[0026]FIG. 7B is a diagram depicting an output pulse from the droopcompensation circuit of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

[0027] Turning now to the drawings and referring first to FIG. 1, aradiation therapy device 10 of a type which may incorporate features ofthe present invention is depicted. Radiation therapy device 10 isconfigured to deliver a therapeutic beam 12 of radiation from atreatment head 24 toward a treatment zone 18 on a patient 22. Patient 22typically is stationed on a treatment table 16 which is positioned tocomfortably orient patient so that treatment zone 18 is positioned inthe path of beam 12. Treatment head 24 is typically located on a gantry26 which is rotatable about an axis 20 to accurately direct beam 12toward treatment zone 18. Electron, photon or any other detectableradiation can be used for the therapy.

[0028] A linear accelerator (shown as item 100 in FIG. 2) is located ingantry 26 to generate the high power radiation required for the therapy.A control system (not shown), used to control and drive the linearaccelerator and other components of radiation therapy device 10, islocated in a housing 30. This control unit may include, for example apower system such as the unified power system of the present invention.

[0029] Radiation therapy device 10 also includes a central treatmentcontrol unit 40 which is typically located apart from housing 30, gantry26, and patient 22 to protect an operator of control unit 40 (such as aradiation therapist) from radiation. Central treatment control unit 40includes output devices such as at least one visual display unit ormonitor 42 and an input device such as a keyboard 44. Data can be inputalso through data carriers such as a data storage devices or averification and recording or automatic setup system.

[0030] Central treatment control unit 40 is typically operated by thetherapist who administers actual delivery of radiation treatment asprescribed by an oncologist by using the keyboard 44 or other inputdevice. The therapist enters data into control unit 40 that defines theradiation dose to be delivered to the patient, for example, according tothe prescription of the oncologist. The treatment data can also be inputvia another input device, such as a data storage device. Various datacan be displayed before and during the treatment on the screen of themonitor 42.

[0031] As described above, radiation therapy device 10 must operate in adifficult environment requiring high accuracy, reliability,maintainability and safety in a relatively small footprint at a low costof operation. Applicants have discovered an improved, unified powersystem which satisfies these requirements.

[0032] Features of embodiments of the present invention will now bedescribed by referring to FIG. 2, where a linear accelerator 100 isshown which may be used in radiation therapy device 10 (FIG. 1). Linearaccelerator 100 includes a control unit 112 which is used to control theoperation of a power supply 110 and the operation of a powerarchitecture 200. Power supply 110 provides D.C. power to powerarchitecture 200. According to embodiments of the invention, powerarchitecture 200 is a highly reliable, solid state source whichgenerates high voltage, flat-topped D.C. pulses of a few microseconds induration which are used to power a load such as a magnetron or klystron140.

[0033] Power architecture 200 may also be used to generate lower voltagepulses to power an electron gun 110. In operation, pulsed microwavesproduced in magnetron or klystron 140 are injected into an acceleratortube 130 via a waveguide system 150. At the proper instant, electrons,which are produced by electron gun 120, are also pulse injected intoaccelerator tube 130. High energy electrons emerge from accelerator tube130 in the form of a beam of approximately 3 mm in diameter. Theseelectrons can be fed to treatment head 170 as a straight beam or to atreatment head 180 as a bent beam. If the electrons are sent totreatment head 180, the electrons are bent by, for example, a bendingmagnet 160 through a suitable angle (e.g., 270 degrees) betweenaccelerator tube 130 and the target. During treatment, this radiationbeam is trained on a treatment zone of patient 22 lying in the isocenterof gantry 26 of radiation therapy device 10 (FIG. 1).

[0034] Applicants have developed a unified power architecture thatovercomes many of the disadvantages of prior art systems. Embodiments ofthe present invention may be understood by now referring to FIG. 3,where a block diagram of a power architecture 200 according to oneembodiment of the present invention is shown. According to oneembodiment, power architecture 200 is driven by a power supply 110 whichproduces D.C. power. In one embodiment, power supply 110 is a switchedpower supply such as a 208 Volt, three phase supply which produces 0-3kV positive D.C. power at 0-7 Amperes. Power supply 110 supplies D,C.power to power architecture 200 which generates high frequency, flattopped, output pulses to drive a load 300. According to one embodimentof the invention, power architecture 200 is a modular design which maygenerate power for different loads, including, for example a magnetronor klystron. A similar configuration of unified power architecture 200may be used to generate power for loads requiring lower voltage, suchas, for example, electron gun 120.

[0035] In one embodiment, the operation of power supply 110 iscontrolled by control unit 112, although in some configurations, somefeatures of power supply 110 may be manually controlled by an operator(not shown). Control unit 112 also controls operation of powerarchitecture 200 to generate output pulses of a desired frequency andmagnitude to drive load 300 in a manner which delivers an appropriatedose of therapeutic radiation in a radiation therapy device 10 (FIG. 1).

[0036] In one embodiment, power architecture 200 includes a storagesection 210 which is charged by power supply 110 through fractional-turntransformer section 250. When a pulse is needed, control unit 112selectively operates switch section 230 for a period to produce a highfrequency output pulse which is stepped up to a high voltage, highfrequency output pulse by fractional-turn transformer section 250. Theoutput of fractional-turn transformer section 250, in one embodiment, ispassed through a droop compensation circuit 270 to ensure that theoutput pulses are of a high quality, having a substantially flat pulsetop. These high quality, flat topped pulses are used to drive load 300.

[0037] In one embodiment, a reset section 290 is controlled by control112 to selectively deliver a reset current to reset the magnetic core offractional-turn transformer section 250, allowing fractional-turntransformer section 250 to operate at higher frequencies, with improvedrise times as will be discussed further below in conjunction with FIG.5.

[0038] According to one embodiment of the present invention, storagesection 210, switch section 230, and reset section 290 are modular andare formed on printed circuit boards (PCB). A number of differentsections 210, 230, 290, each on a PCB, may be provided based on powerneeds of load 300. Maintenance is simplified and expedited as adefective section may be fixed by simply replacing one or moreindividual PCBs. According to one embodiment of the invention, each PCB(which contains one “branch” or “section” of power architecture 200 aswill be discussed further below), may be formed with identicalcomponents. Manufacturing, maintenance, stocking, and installation isthus simplified by requiring that only a single configuration of PCB beproduced for a given design.

[0039] According to a further embodiment of the present invention, eachbranch, including fractional-turn transformer section 250, is formed ona single PCB, further facilitating maintenance and supply. Applicant hasfound that lower power systems are suited for this embodiment. Forexample, the power architecture for use with an electron gun such as theelectron gun 120 of FIG. 2, may be implemented using features of thepresent invention where each branch, including section 250, may beformed on an individual PCB. Those skilled in the art, upon reading thisdisclosure, will realize that other types of loads may also lendthemselves to such use.

[0040] Further, as will be described in more detail below, the output ofpower architecture 200 may be increased or decreased by simply adding orremoving PCBs. The output of power architecture 200 may also be adjustedby applying different control signals from control 112 to each branch ofpower architecture 200. For example, in one embodiment, each branch maybe controlled to generate an output pulse at a slightly different time,causing the overall output pulse of the power architecture 200 to riseat a different rate (e.g., an electronic dV/dt adjustment). The resultis a highly-reliable, easily maintainable and modular power system whichincreases the reliability and accuracy of radiation therapy deviceswhile reducing the cost of operation and ownership. Because the designis a solid state design, the size of the system is relatively compact.Further still, as will be discussed further below, the system does notrequire dangerous high voltage and high current inputs, reducing thecost and complexity of other components, including power supply 110.

[0041] Further details of power architecture 200 will now be describedby referring to FIG. 4. As can be seen more clearly in FIG. 4, powerarchitecture 200 includes a number of branches (each branch includingcomponents designated by the lower-case letters a-n). Each branch may beformed on a single PCB. In one embodiment of the present invention, eachbranch includes storage 210, a switch 230, a reset circuit 290, atransformer core 250, and one or more primary winding set(s) 252. Powersupply 110 provides power to each of the branches of power architecture200, charging storage devices 210 a-n. High frequency pulses aregenerated by selectively operating switches 230 a-n resulting in pulsedpower being provided to each of the primary winding sets 252 a-n oftransformer cores 250 a-n. Further details of one currently-preferredembodiment of storage 210 and switch 230 will be described further belowin conjunction with a discussion of FIG. 5.

[0042] In a currently-preferred embodiment, the transformer cores 250have a commonly-wound secondary to cause the transformer to operate as afractional-turn transformer. The windings 254 o-z of the secondarywinding set generate an output pulse with a voltage approximately equalto the sum of the input voltages at the primary winding sets. In anembodiment designed to provide power to a klystron, five (5) transformercores 250 are used, each having three (3) primary winding sets and acommonly-wound secondary set having fifty (50) windings 254. In anembodiment designed to provide power to a magnetron, two (2) transformercores 250 are used, each having three (3) primary winding sets and acommonly-wound secondary set having fifty-four (54) windings 254.

[0043] The pulse output from the secondary is a high voltage, lowcurrent output pulse formed from a low voltage D.C. power input frompower supply 110. The selection of materials and structure oftransformer cores 250 and winding sets 252, 254 depend on theapplication, input characteristics and output characteristics and areknown to those skilled in the art. In one embodiment, transformer cores250 are formed of a material such as a magnetic alloy which has highsaturation induction, low core loss, and high B-H squareness. Accordingto one embodiment of the present invention, reset circuit 290 a-n isprovided to deliver a reset current to reset the magnetic material ofeach transformer core.

[0044] According to one embodiment of the present invention, the outputpulse from windings 254 o-z of the secondary winding set is sent througha droop compensation circuit 270 to ensure that the output pulse shapeis of good quality (i.e., a relatively flat pulse top) to drive load300. High frequency sources such as klystrons and magnetrons requireinput pulses of relatively good quality, particularly where the highfrequency sources are used in medical radiation therapy environments.

[0045] According to one embodiment of the invention, a single droopcompensation circuit is provided at the output of the powerarchitecture. Applicants have discovered that such a configurationprovides an improved quality output pulse as compared to systems whichprovide droop compensation on pulses input to the primary side of thetransformer. Further, according to one embodiment of the invention,space on individual PCBs is conserved and power dissipation is reducedby not utilizing droop compensation circuits on the primary side of thetransformer. The result is a system which provides an improved qualityhigh frequency pulse to drive a variety of loads.

[0046] According to one embodiment of the present invention, the numberof branches, the number of transformer cores 250, and the number ofsecondary windings 254 are based on requirements of load 300. Forexample, in one embodiment, each branch is designed to produceapproximately four (4) MW of peak output power per branch. Portions ofeach branch may be produced on a separate PCB. Therefore, as an example,to configure unified power architecture to drive a klystron (such as aklystron requiring peak power of approximately 19.2 MW), five (5)branches, transformer cores 250 (e.g., 250 a, 250 b, 250 c, 250 d, and250 e) with commonly-wound secondaries are used to produce an outputpulse of 160 kV at 120 Amperes with a pulse width of between 5 to 8μSeconds at a cycle rate of between 100 and 400 Hertz (e.g., averagepower of approximately 20 kW).

[0047] In this example, the unified power architecture may be configuredto drive a different load such as a magnetron, by providing a differentnumber of branches. For example, where the load to be driven is amagnetron (such as a magnetron requiring peak power of approximately 6MW), two (2) branches and transformer cores 250 (e.g., 250 a and 250 b)are used to produce an output pulse of 50 kV at 110 Amperes with a pulsewidth of between 5 to 8 μSeconds at a cycle rate of between 100 and 400Hertz (e.g., an average power of approximately 6-8 kW). Upon readingthis disclosure, those skilled in the art will now recognize thatfeatures of embodiments of the present invention may be used to developa modular power supply for any of a number of different loads. Accordingto one embodiment of the present invention, different loads can beaccommodated by simply adding or removing PCBs having individualbranches of the invention.

[0048] Referring now to FIG. 5 a schematic diagram is shown whichdepicts a portion of power architecture 200 according to an embodimentof the present invention. In particular, FIG. 5 depicts one branch ofpower architecture 200 of FIG. 4. According to one embodiment of thepresent invention, each branch of power architecture 200 is similarlyconfigured and may be formed on a separate PCB.

[0049] Each branch is coupled to receive input power from power supply110. An input circuit including an inductor 504 and resistor 506 may beused to isolate each branch from power supply 110 as is known to thoseskilled in the art. In one example embodiment, where power supply 110 isa switched 208 Volt, three phase supply producing approximately 1850 kVpositive D.C. power at 0-11 Amperes, inductor 504 is a 600 μH inductorand resistor 506 is a 5 ohm resistor. Those skilled in the art willrecognize that these values are presented to illustrate an embodiment ofthe invention, and that other components and values may be selected asneeded to isolate each branch from power supply 110.

[0050] Operation of power supply 110 is controlled by control 112, whichalso controls the operation of an Insulated Gate Bipolar Transistor(IGBT) 508 by applying a control signal to a control pad 510 of IGBT508. IGBT 508 is switched on or off based on a voltage applied to a gateof IGBT through control pad 510. Power supply 110 is operated to chargecapacitor 522 through primary winding set 252 a. Pulses are generated byselectively switching IGBT 508 on and off (i.e., placed in a conductingor non-conducting state) to couple capacitor 522 to primary winding set252 a. A pulse may be generated by operating control 112 to command thegate drive circuit 510 to switch IGBT 508 to a conducting state. IGBT508 then connects the charged capacitor 522 to primary windings 252 a,producing a high voltage output pulse at the secondary. When the desiredpulse width is reached, control 112 causes gate drive circuit 510 toplace IGBT 508 in a non-conducting state to end the pulse. In theterminology of FIG. 3, IGBT 508 operates as the switch 230, andcapacitor 522 operates as the storage 210. Embodiments of the presentinvention allow the production of an output pulse which has a pulsewidth that is continuously variable. The pulse width may be controlledby control 112. This provides great variability and precision intreatment control for radiation therapy devices which are driven usingpower architecture 200. Further, because the variable pulse width isgenerated using IGBT switching action, pulse forming networks are notneeded, and the impendence matching problems associated with those pulseforming networks are eliminated.

[0051] In one embodiment, control 112 is operated to cause IGBT 508 toswitch every 4 to 7.5 msec, causing a high frequency output pulse to begenerated at primary winding set 252 a. The frequency at which IGBT 508is operated is selected based on the treatment plan established forradiation therapy device 10 (FIG. 1) (e.g., a treatment plan calling fora very high frequency dose to be delivered to a patient may require thatIGBT 508 be switched at a higher frequency). Applicants have found thatIGBTs produced by the European Power-Semiconductor and Electronics Co.(EUPEC) of Warstein Del. (such as their IGBT model FZ 1200 R33 KF2) aresuitable for use in embodiments of the invention, although othermanufacturers and models may also be suitable so long as the IGBT hassuitably high turn on and turn off times (e.g., approximately 100-300ηsec).

[0052] Capacitor 522 is a large storage capacitor, in one embodiment a100 μF capacitor capable of producing a charge of approximately 2000V.Those skilled in the art will appreciate that rather than using a singlecapacitor, a bank of more than one capacitor may be provided to achievethe functionality of capacitor 522. Preferably, capacitor 522 isselected to deliver high current surges at a high repetition frequency.Further, capacitor 522 is preferably selected to provide a long servicelife. Embodiments of the present invention provide a greater servicelife by allowing capacitor 522 to remain charged after each pulse.Further, embodiments of the present invention allow use of lower chargevoltages by spreading the pulsed voltage value among several (e.g., fivefor a klystron load) branches of the power architecture 200. This allowsthe use of lower voltage components (such as IGBT 508 and power supply110) and may avoid the need to immerse the entire power supply 110 andswitching electronics in oil (ensuring easier and cheaper maintenance).

[0053] In some embodiments of the present invention, a snubber circuit520 is provided to dissipate excess current which results when IGBT 508is operated at the end of a pulse. At this time, current flowing towardIGBT 508 is diverted to snubber circuit 520. Current flowing towardcapacitor 522 is diverted to a reset circuit 530 (which will bediscussed in detail below). Both snubber circuit 520 and reset circuit530 are designed to allow the currents to return to zero withoutgenerating excessive voltage on the IGBT.

[0054] In one embodiment, snubber circuit 520 consists of a resistor 514coupled to a capacitor 516, both in parallel with a diode 518. Each ofthese components is selected to avoid placing IGBT 508 in an overvoltagecondition by controlling the rate of current discharge which occurs whenIGBT 508 is opened. In one embodiment, a relatively large current (up toapproximately 2000 A) attempts to discharge across IGBT 508 when theIGBT 508 is opened. Components of snubber 520 are selected to provide adischarge path for this current. In one embodiment, resistor 514 is a 25ohm resistor (a value selected based on the desired rate of discharge),capacitor 516 is a 0.3 μF capacitor, and diode 518 is a 3500V, 120 Aassembly.

[0055] In one currently-preferred embodiment, a separate snubber circuit520 is not necessary for the operation of power architecture 200,thereby reducing the cost and power dissipation associated with eachPCB. Instead, IGBT 508 is be selected which has a control feature thatcan be set to operate safely in short circuit conditions, therebyallowing a discharge path to ground across the IGBT. Gate control 510may be selected and operated to detect and limit the maximum amount ofcollector current, thereby managing short circuit conditions. Gatecontrol 510 preferably also is selected and operated to turn off“normal” pulses to eliminate the need for snubber 520. This isaccomplished by controlling the speed at which IGBT current isterminated at the end of each pulse during normal operation. Preferably,the speed at which IGBT current is terminated is selected to besufficiently slow to reduce IGBT voltage. IGBT 508 is further protecteddue to the low inductance interconnections provided by the PCB and thenormal operation of the reset circuit 530. Other techniques fordischarging current built up as IGBT 508 switches off may also be used.

[0056] In one embodiment, a reset circuit 530 is also provided. Resetcircuit 530 performs several functions, one of which is to set the coreof transformer 250. Those skilled in the art appreciate that,particularly to achieve high frequency operation, magnetic materialsused as transformer cores often require the application of a reversevoltage. Many systems provide a small, separate power supply to performthis reset function. According to one embodiment of the presentinvention, reset circuit 530 is used, reducing the overall cost of thepower architecture 200 while achieving high performance in highfrequency conditions.

[0057] According to the invention, reset circuit 530, as will bediscussed further below, also provides some protection to the system inthe event that IGBT 508 fails to function properly. Further, resetcircuit 530 may also be used to assist in the production of a clean,well-formed pulse as an output. Each of these features of reset circuit530 help to ensure that power architecture 200 performs efficiently andreliably in high frequency conditions.

[0058] Reset circuit 530 includes, in one embodiment, a resistor 524coupled with IGBT 526. IGBT 526 is coupled to a reset capacitor 522. Adiode 530 is coupled to provide a path to reset capacitor 532. A controlpad 528 coupled to a gate of IGBT 526 is operated to switch IGBT 526 tocontrol the generation of charging current from reset capacitor 532. Inone embodiment, control pad 528 of IGBT 526 is coupled to control 112 tosynchronize the release of charging current after discharge of currentfrom capacitor 522.

[0059] Operation of IGBT 526 may be controlled by control 112 toprogress as follows. During the time period that IGBT 508 is operated togenerate an output pulse, reset circuit 530 is inactive. At the end ofthe pulse (IGBT 508 is caused to open by control 112), diode 520 andreset capacitor 532 provide a pathway for current flowing in thetransformer 250. This results in a voltage in reset capacitor 532 thatmay reach (in one embodiment) approximately 100V after several pulses.During the time between pulses (in one embodiment, more thanapproximately 2000 μsec), IGBT 526 in reset circuit 530 is switched bycontrol 112 to connect the charged reset capacitor 532 to primarywinding set 252 a. The resulting current is directed in a way that“resets” or re-magnetizes core 250a to a state that is favorable for thenext pulse. By controlling the reset pulse width (via control 112operating gate control 528), the voltage across IGBT 532 is regulated toa constant value. Resistor 524 operates to limit the value of resetcurrent flowing between reset capacitor 532 and primary winding set 252a.

[0060] In one embodiment, resistor 524 is a 10 ohm resistor, and resetcapacitor 532 is a 100 μF capacitor selected to provide a small chargingcurrent to reset transformer core 250 a. Applicants have discovered thatuse of reset circuit 530 provides a number of useful advantages in theoperation of power architecture 200. For example, reset circuit 530captures and stores the magnetic energy remaining in the transformercore after each pulse. The energy is stored in capacitor 532 andreleased back to the system between pulses in a way that: (a) controlsthe backswing voltage on the load; (b) resets the magnetic core for thenext pulse; and (c) shapes the fall time of the main output pulse.

[0061] Those skilled in the art will appreciate that selection ofparticular components of reset circuit 530 will depend upon the maximumand minimum operating frequencies as well as the size of the chargingcurrent required to reset the transformer core.

[0062] Operation of power supply 110, storage capacitor 522 and IGBT 508results in the generation of a high frequency pulse presented to primarywinding set 252 a. A higher voltage, stepped-up output voltage (Vout) isproduced at secondary winding set 254. Because more than one branch isoperated in parallel (e.g., as shown in FIG. 4), with a commonly woundsecondary (i.e., operating as a fractional-turn transformer), Voutincludes components from each of the branches. Control 112 ensures thatIGBTs 508, 526 are switched, and charging currents for each of thebranches are applied at the same time and at the same frequency,ensuring that Vout is a high voltage output pulse of a desiredfrequency.

[0063] Embodiments of the present invention provide features which helpto ensure that failure of a branch (e.g., as a result of failure of IGBT508) does not cause the entire power architecture 200 to fail. In oneembodiment, reset circuit 530 functions to assist in an open circuitfailure of IGBT 508. Without features of the present invention, an opencircuit failure would result in the failed branch denying the otherbranches a convenient current pathway. According to embodiments of theinvention, this failure is prevented. For example, if IGBT 508 of abranch of power architecture 200 fails as an open circuit, reset circuit530 will continue to provide some current (via reset capacitor 532) toprimary winding set 252, allowing the other branches of the powerarchitecture 200 to pass their primary currents through the fractionalturn transformer. If IGBT 508 fails as a short circuit, the system willcontinue to function with the other branches using the failed IGBT tocarry their current.

[0064] The resulting output voltage (Vout) in such a condition will bereduced by an amount equal to the percentage contribution otherwiseprovided by the failed branch (e.g., if five input branches aretypically used to drive a klystron, and one of the branches fails, theoutput pulse will be reduced by approximately 20% in magnitude). Thoseskilled in the art will recognize that appropriate monitoring circuitryand devices may be used to detect such a reduction, allowing the powerarchitecture 200 to shut down in such an event.

[0065] Each branch of power architecture 200 includes similar componentssized to generate a desired Vout across secondary winding set 254 o-z ofthe transformer. According to the invention, each branch is formed ofthe same design and is formed on PCB, allowing simple and quickmaintenance. A defective branch may be fixed by simply replacing thebranch with another PCB. Further, the output of power architecture 200is scalable by simply adding or removing individual branches.Maintenance planning is thus simplified by requiring stocking of asingle PCB design for various types of loads.

[0066] In one embodiment, where the load is a magnetron, two (2)branches are used which can produce a Vout of approximately 50 kV at 110Amperes. In another embodiment, where the load is a klystron, five (5)branches are used which can produce a Vout of approximately 160 kV at120 Amperes with pulse width of approximately 4-7.5 μsec at a frequencyof 100 to 400 Hz. According to one embodiment of the invention, for bothloads (the magnetron and klystron), the same branches may be used; theklystron simply utilizes more branches than the magnetron. The result isan improved power architecture which is modular, reliable, and highlyaccurate.

[0067] According to one embodiment of the invention, the output pulsegenerated at Vout is improved by passing the output pulse through adroop compensation circuit such as the circuit depicted in FIG. 6. Asshown in FIG. 6, droop compensation circuit 270 is coupled to receivethe output pulses from the secondary windings around the transformer(s)(which pulse is designated as Vout) and delivers a pulse of improvedquality to load 300 (the improve pulse is designated as V'out). In oneembodiment, where the droop compensation circuit 270 utilizes passivecomponents, an inductor 272 having an inductance of approximately 90 μHis coupled in parallel to a series connected resistor 274 and capacitor276. Resistor 274, in one embodiment, has a resistance of 25 Ohms whilecapacitor 276 has a capacitance of 30 nF. The values of these componentsare selected to trim voltage off a leading edge of a pulse andredistribute it along the remainder of the pulse, resulting in aflattened pulse top. Those skilled in the art will recognize that thesizes of components used will vary on the frequency and magnitude ofpulses to be generated by the system.

[0068] Referring now to FIG. 7A, a pulse diagram is shown which depictsa waveform of Vout without droop compensation. As depicted, the leadingedge of the pulse at t₀ is slightly greater than the trailing edge att₁. By passing the pulse through droop compensation circuit 270, theoverall quality of the pulse is improved. As shown in FIG. 7B, theimproved output pulse (shown as V'out) has a substantially flat pulsetop from the leading to the trailing edge. By providing a single droopcompensation circuit at the output of the secondary of thefractional-turn transformer, Applicants have discovered that the overallquality of the output pulse is improved without excess cost andcomplexity. Droop compensation circuit 270, in combination with otherelements of the invention, also provides other operational advantages.For example, use of droop compensation circuit 270 allows the use offewer and smaller energy storage capacitors (capacitor 522 of FIG. 5),saving space and cost. Fewer and smaller energy storage capacitors alsoensures that less damage is suffered in the event of a short circuitcondition. Droop compensation circuit 270 assists in the event of ashort circuit by limiting the magnitude and rate of current rise,allowing other circuits to detect that a short circuit condition hasoccurred. Droop compensation circuit 270 ensures that a clean,well-formed pulse is output despite the use of fewer and smaller storagecapacitors. Those skilled in the art will recognize, upon reading thisdisclosure, that other types of droop compensation circuits may be usedwith embodiments of the present invention. Preferably, as with theabove-described embodiment, any such circuits are provided at the outputof the secondary of the fractional-turn transformer.

[0069] Although the present invention has been described with respect toa preferred embodiment thereof, those skilled in the art will note thatvarious substitutions may be made to those embodiments described hereinwithout departing from the spirit and scope of the present invention.

What is claimed is:
 1. A unified power architecture for generating highfrequency switched power to a load, comprising: a number of inputbranches, each input branch receiving direct current power from a powersource, each input branch comprising a switch device coupled to astorage device for generating input pulses; and a transformer includinga primary winding set receiving said input pulses; an output sectionincluding a plurality of windings around a secondary of each transformerof each of said input branches, said output section generating an outputpulse including components of each of said input pulses; and a droopcompensation circuit, coupled to receive said output pulse, flattening atop of said output pulse.
 2. The unified power architecture of claim 1,wherein said number of input branches is two.
 3. The unified powerarchitecture of claim 1, wherein said number of input branches isselected based on a load to be driven by said output pulse.
 4. Theunified power architecture of claim 3, wherein said load is selectedfrom one of a magnetron, a klystron, and an electron gun.
 5. The unifiedpower architecture of claim 1, wherein each of said number of inputbranches is formed on a single printed circuit board (PCB).
 6. Theunified power architecture of claim 1, wherein said switch device andsaid storage device are formed on a single printed circuit board (PCB),each of said PCBs being interchangeable among input branches.
 7. Theunified power architecture of claim 5, wherein each of said number ofinput branches is interchangeable.
 8. The unified power architecture ofclaim 1, further comprising a control circuit, coupled to each of saidnumber of input branches to: selectively control said switch devices togenerate a desired frequency of said input pulses.
 9. The unified powerarchitecture of claim 1, wherein said droop compensation circuit isadapted to reduce the magnitude of a leading edge of said output pulse.10. The unified power architecture of claim 8, wherein each of saidswitch devices include an IGBT whose gate is coupled to said controlcircuit.
 11. The unified power architecture of claim 10, wherein saidIGBT is further coupled to a snubber circuit to discharge excess currentwhen said IGBT is switched between a conducting and non-conductingcondition.
 12. The unified power architecture of claim 10, wherein awidth of said output pulse is varied by said control circuit.
 13. Theunified power architecture of claim 10, wherein a rise time of saidoutput pulse may be varied by said control circuit.
 14. The unifiedpower architecture of claim 1, wherein said number of input branches istwo and said unified power architecture generates peak power output ofeight mega Watts.
 15. The unified power architecture of claim 1, whereinsaid number of input branches is five and said unified powerarchitecture generates peak power output of twenty mega Watts.
 16. Aunified power architecture for use in a radiation therapy device,comprising: a control unit; a power supply, selectively controlled bysaid control unit to generate DC power; a plurality of input powersections, each input power section coupled to receive DC power from saidpower supply and including a switch device coupled to a capacitor forgenerating input pulses, said switch device selectively controlled bysaid control unit; a transformer including at least one primary windingset receiving said input pulses; an output section including a pluralityof windings around a secondary of each transformer of each of said inputpower sections, said output section generating an output pulse includingcomponents of each of said input pulses; and a droop compensationcircuit, coupled to receive said output pulse, and adapted to generatean improved output pulse with a flatter pulse top than said outputpulse.
 17. The power architecture of claim 16, wherein two input powersections are provided and said output pulse is provided to a magnetron.18. The power architecture of claim 16, wherein five input powersections are provided and said output pulse is provided to a klystron.19. The power architecture of claim 14, wherein each of said pluralityof input power sections are formed on a separate printed circuit board.20. A power architecture, comprising: a control unit; a DC power source,selectively operated by said control unit to generate DC power; aswitch, coupled to receive input power from said DC power source andelectronically operated between an on and an off state by said controlunit; a storage device which generates a first pulse when said switch isoperated between said on and off states; a transformer having a primarywinding set and a secondary winding set, said primary winding setreceiving said first pulse, said secondary winding set also coupled to asecondary of at least a second transformer and producing a second pulsehaving a higher voltage than the first pulse; and a droop compensationcircuit coupled to receive said second pulse and adapted to produce athird pulse having a pulse top flatter than a pulse top of said secondpulse.
 21. A method for driving a high frequency load, comprising:generating DC power; providing said DC power to a number of inputbranches, each input branch generating input pulses by operating aswitch device in conjunction with a storage device; applying said inputpulses to a primary winding set of a transformer core; generating anoutput pulse from a secondary winding set of a transformer of each ofsaid input branches, said output pulse including components of each ofsaid input pulses; and generating an improved output pulse by flatteninga pulse top of said output pulse.
 22. A unified power architecture forgenerating high frequency switched power to a load, comprising: a numberof modular input branches, each input branch receiving direct currentpower from a power source, each input branch comprising a switch devicecoupled to a storage device for generating input pulses; a transformerincluding a primary winding set receiving said input pulses; and anoutput section including a plurality of windings around a secondary ofeach transformer of each of said input branches, said output sectiongenerating an output pulse including components of each of said inputpulses.
 23. A unified power architecture for generating high frequencyswitched power to a load, comprising: a number of modular inputbranches, each modular input branch receiving direct current power froma power source, each modular input branch comprising a switch devicecoupled to a storage device for generating input pulses and atransformer including a primary winding set receiving said input pulses;and an output section including a plurality of windings around asecondary of each transformer of each of said input branches, saidoutput section generating an output pulse including components of eachof said input pulses.